1. Field of the Invention
The invention relates to a knocking control apparatus and knocking control method for an internal combustion engine including a variable valve mechanism which can vary a valve characteristic of at least one of an intake valve and an exhaust valve.
2. Description of the Related Art
An internal combustion engine is known, which includes a variable valve mechanism that can vary a valve characteristic, for example, opening/closing timing and a lift amount of an intake valve and/or an exhaust valve, according to an engine operating state.
Meanwhile, in an internal combustion engine, a knocking determination as to whether knocking occurs is generally performed. On the basis of the result of the knocking determination, knocking control is performed to adjust an engine control amount such as ignition timing. This knocking determination is performed using a knock sensor which is provided in a cylinder block or the like for detecting vibrations. On the basis of an output signal from the knock sensor after ignition at each cylinder, it is determined whether knocking occurs.
In the internal combustion engine including the aforementioned variable valve mechanism, an occurrence mode of seating noise, that is, a mode in which seating noise occurs when the intake valve or exhaust valve is closed is changed due to a change in the valve characteristic. In other words, for example, occurrence timing, a vibration level, or a vibration frequency of the seating noise is changed due to the change in the valve characteristic. As a result, this seating noise may be detected as knocking. Accordingly, in a conventional knocking control apparatus for an internal combustion engine including a variable valve mechanism, a mode of the knocking determination is changed according to the change in the valve characteristic so as to prevent such seating noise from being detected as knocking.
For example, in a knocking control apparatus disclosed in Japanese Patent Application Publication No. JP(A)2002-221054, vibration frequencies of an engine that are detected by a knock sensor are subjected to a filtering process so that a vibration frequency indicating occurrence of knocking is extracted. The vibration frequency that is extracted during this filtering process (hereinafter, referred to as “knocking detection frequency”) is changed according to the change in the valve characteristic. Thus, a vibration frequency which is caused due to seating of the valve is prevented from being misjudged as the vibration frequency of knocking. Also, various proposals are made, in which a determination period or a determination level is changed according to the change in the valve characteristic.
Meanwhile, when the valve characteristic of the intake valve or the exhaust valve is changed, an air-intake state and the like are changed. Therefore, a combustion state of an air-fuel mixture may be also changed. Thus, an occurrence mode of knocking, for example, occurrence timing of knocking, a vibration level of knocking, or a vibration frequency caused by knocking may be also changed. In the aforementioned conventional knocking control apparatus, although it is possible to appropriately reduce influence of the seating noise of the intake valve or exhaust valve on the knocking determination, it is not possible to cope with a change in the occurrence mode of knocking, which is caused by the change in the combustion state. Accordingly, in the case where the valve characteristic is changed, it may be determined that knocking occurs even when knocking does not occur. Also, it may be determined that knocking does not occur even when knocking occurs. That is, reliability of the result of the knocking determination performed in the internal combustion engine including the variable valve mechanism may be reduced, which may adversely affect adjustment of the engine control amount. Therefore, further improvement is required.